This invention relates generally to the completion of wellbores. More particularly, this invention relates to new and improved methods and devices for completion, deepening, fracing, reentering and plug milling of the wellbore. This invention finds particular utility in the completion of horizontal wells. Notwithstanding previous attempts at obtaining cost effective and workable horizontal well completions, there continues to be a need for increasing horizontal well departure to increase, for example, unconventional shale plays—which are wells exhibiting low permeability and therefore requiring horizontal laterals increasing in length to maximize reservoir contact. With increased lateral length, the number of zones fractured increases proportionally.
Most of these wells are fractured using the “Plug and Perf” method which requires perforating the stage nearest the toe of the horizontal section, fracturing that stage and then placing a composite plug followed by perforating the next stage. The process is repeated numerous times until all the required zones are stimulated. Upon completing the fracturing operation, the plugs are removed with a positive displacement motor (PDM) and mill/bit run on coiled tubing (CT). As the lateral length increases, milling with coiled tubing becomes less efficient, leading to the use of jointed pipe for removing plugs.
Two related reasons cause this reduction in efficiency of the CT. First, as the depth increases, the effective maximum weight on bit (WOB) decreases. Second, at increased lateral depths, the coiled tubing is typically in a stable helical spiral in the wellbore. The operator sending the additional coiled tubing (and weight from the surface) will have to overcome greater static loads leading to a longer and inconsistent transmission of load to the bit. The onset of these two effects is controlled by several factors including; CT wall thickness, wellbore deviation and build angle, completion size, CT/completion contact friction drag, fluid drag, debris, and bottom hole assembly (BHA) weight and size. CT with an outer diameter less than 4 inches tends to buckle due to easier helical spiraling, thus increasing the friction caused by increased contact surface area along the wall of the bore hole. CT outer diameters greater than 4 inches are impractical due to weight and friction limitations. Friction drag is a function of CT wall thickness and diameter, leaving end loads as one of the variables most studied for manipulation to achieve better well completion.